There is known a technology in which a rotary-type throttling device is provided in an intake passage for introducing fresh air into a combustion chamber of an internal combustion engine, so that the combustion chamber is supplied with fresh air when a rotary valve is opened, and in which an auxiliary intake passage is provided in bypassing relation to the rotary valve for supplying fresh air to the intake passage downstream of the rotary valve while the engine is idling with the rotary valve being closed (see, for example, Patent Document 1).